I industries processing lignocellulosic biomass material, such as wood chips, herbaceous plants, straw, bagasse etc., transport arrangements are of high importance. The feeding of the lignocellulosic biomass material to be processed or have been processed depends on many factors. The nature of the lignocellulosic biomass material, such as fibre length, water content, density etc., give rise to different requirements as well as the properties of the treatments performed before and/or after the transporting.
Plug screw feeders, in which a rotating screw transports the material forward is a commonly used type of feeder in fibrous material processing, in particular for lignocellulosic biomass material. At the same time as the material is fed to a next stage, the screw and associated screw pipe or housing also exercises a volumetric compression function. The lignocellulosic biomass material is typically provided through an inlet opening in the side of the plug screw feeder, close to the rear end.
In a basic set-up and in particular when the lignocellulosic material consists of wood chips or other denser materials, the lignocellulosic biomass material can simply be provided through the inlet opening by gravitational forces. However, when handling straw and other materials of low density and if a well reproducible high density is requested at the front end of the plug screw feeder, forced feeding may be employed. Examples of such force-feeding can be found in e.g. the U.S. Pat. No. 5,320,034 or in the published International patent application WO 2013/126007 A1. The material leaving the front end is in certain applications requested to have a high density. For instance, when feeding lignocellulosic biomass material into a digester, operating at high pressures, it is requested that the front end material volumes themselves provides a sealing for the high pressure. There is thus a need for a high, uniform and reliable compression of the lignocellulosic biomass material during the feeding through the plug screw feeder.
One problem that might occur using prior art plug screw feeders, in particular for fibrous material having relatively long fibres, e.g. in lignocellulosic biomass material, is that the operation of the plug screw feeder becomes irregular. The actual feeding into and within the plug screw feeder seems to be provided intermittently or jerkily. The plug screw feeder tends to be exposed for repeated short periods of very high stress and the overall compression result becomes deteriorated. It is e.g. common that the achieved compression in the material presented at the front end differs considerably in a circumferential direction. Also the power requirements of the plug screw feeder becomes irregular, which require high safety margins.
There is thus a need for an improved feeding arrangement suitable for all kinds of lignocellulosic biomass material.